Notes on Energy | Grade 8 > Science > Work, Energy and Power | KULLABS.COM

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The capacity of a body to do work is called the energy of the body. The SI unit of energy is joule (J). Energy can exist in various forms. Different forms of energy are:

1. Mechanical energy: Energy by the virtue of which a body can do some mechanical work directly, is called mechanical energy. The common forms of the mechanical energy are:

• Kinetic energy:The energy possessed by a body due to its motion is called kinetic energy. Kinetic energy (K. E) of a body having mass m, and velocity v, is equal to equation That is, K. E = mv2

• Potential energy:The energy possessed by a body due to its position or configuration is called potential energy. Potential energy (P.E) of a body is P.E = m x g x h

Where, m is a mass of the body, g is the acceleration due to gravity and h is height.

2. Electrical energy: A form of energy created by the movement of electrons is called electrical energy. I.e. hydropower, generator, etc.

3. Light energy: Light is also form of energy that makes things visible. We use light in the different process like lightening the houses, running industries, etc. Plants use energy to make food.

4. Heat energy: Heat energy is obtained by burning combustible substances like coal, gas, kerosene, firewood, etc. We use heat energy to keep warm and to cook food.

5. Sound energy: Sound energy is also a form of energy which is produced by vibrations. We hear the sound when the vibrations travel through the air around it to vibrate.

6. Chemical energy: The energy stored in the matter such as the energy stored in food and fuels, is chemical energy.

7. Nuclear energy: The neutrons and protons exert an attractive force on each other when present in the nucleus. The energy associated with them is called nuclear energy.

8. Magnetic energy: The energy can also be produced by a magnet. A magnet is the source of energy. Electricity can be produced with the help of the magnet. It is more powerful and also can be used to lift a heavy weight.

#### Transformation of energy

Conversion of energy from one form to another form is called the transformation of energy.

When a candle burns, chemical energychangesinto light and heat energy. When a fan is switched on, it converts an electrical energy into mechanical energy. Similarly, when a lamp glows, it changes electrical energy into light and heat energy. These examples show that one type of energy can be changed into another type.

Examples:

1. A steam engine changes heat energy into mechanical energy.
2. Solar car converts solar energy into mechanical energy.
3. A microphone converts sound energy into electrical energy.

#### Conservation of energy

Energy can neither be created nor be destroyed but can only be changed from one form to other. This is called the principle of conservation of energy.

• The capacity of a body to do work is called the energy of the body. The SI unit of energy is joule (J). Energy can exist in various forms.
• Conversion of energy from one form to another form is called transformation of energy.
• This is called the principle of conservation of energy.
.

### Very Short Questions

 s.n magnetic energy chemical energy 1 It is produce from magnet. It is due to a chemical reaction. 2 Electricity can be produced with the help of magnet. This energy is released in the form of electrical energy. 3 It is used in radio, telephone, and telegraph etc. Store in matter such as the energy store in food and fuel.

The energy possessed by a body due to its position or configuration is called potential energy. For example, a raised stone has potential energy and can do the work and a stretched rubber band has potential energy because of their stretched condition.

A flying bird possesses both potential and kinetic energies as it is at a certain height above the ground and moving with the certain velocity.

A man is standing at the rest on a moving car does not possess kinetic energy.

Chemical energy is form of energy that the food contains.

Kinetic energy depend on following factors:

• The kinetic energy of a body is directly proportional to the mass of the moving body.
• The kinetic energy of a body is direct proportional to square of velocity of body.

Conversion of energy from one form to another form is called the transformation of energy. Examples of conservation of energy are:

1. A steam engine changes heat energy into mechanical energy.
2. Solar car converts solar energy into mechanical energy.
3. A microphone converts sound energy into electrical energy.

Energy can neither be created nor destroyed but can only be changed from one form to other. This is called the principle of conservation of energy.

\begin{align*} \text{mass}\:(m) &= 65\: kg \\ \text {velocity} \: (v) = 10 km/hr = \frac {10\times 1000 m}{1 \times 60\: \text {min}\times 60\: \text {seconds}}\\ &= 0.2777\: km/hr \\ \text {kinetic energy} (K.E) &= ?\\ \text {We know} \\ K.E. &= \frac {1}{2}mv^2 \\ &= \frac 12 \times 65 \times (0.277)^2 \\&= 2.49 \: \text {joules} \\ \end{align*}

\begin{align*} \text {Given,} \\ \text {mass} \:(m) &= 10 kg \\ \text {height from the ground} \: (h) &= 10\: m \\ \text {acceleration due to gravity} \:(g) &= 9.8 ms^2 \\ \text {potential energy}\: (P.E.) &= ? \\ P.E. &= m\times g\times h \\ &= 10 \times 9.8 \times \: 10 \\ &= 980\: \text {joule} \\ \end{align*}

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• ### One joule is the amount of work done when ______.

one newton force covers a distance of 1 m

none of these

one newton force does not covers any distance

one newton force covers distance of 2 m

• ### Energy is defined as ______.

the rate of doing work

the capacity to do work

the average rate of work

the amount of work

• ### Which statement is true for electrical energy?

It is a form of energy produced by vibrations

It is the energy associated with neutrons and protons

It is a form of energy created by the movement of electrons

It is the energy possessed by a body due to its motion

• ### A rubber band zinged from your finger has ______.

chemical energy

heat energy

potential energy

kinetic nergy

• ### Which of the following are the types of energy?

heat energy

mechanical energy

electrical energy

• ### Kinetic and Potential energies are common forms of ______.

mechanical energy

heat energy

nuclear energy

electrical energy

• ### The energy possessed by the moving object is _______.

mechanical energy

heat energy

kinetic energy

electrical energy

• ### Kinetic energy of a body depends on ______.

mass of the body

none of the answers are right

acceleration due to gravity

height raised from the ground

• ### If the velocity of the body is doubled, then the kinetic energy of the body ______.

increases thrice the original

increases four times the original

does not increase at all

increases twice the original

• ### Which one of the following  is the use of kinetic energy?

penetration of the bullet into the target.

all of the answers are right.

use of hammer to push the nail into the wood.

use of running water to run the water mills.

• ### Energy possessed by a body by the virtue of its position or configuration is called ______.

electrical energy

heat energy

mechanical energy

potential energy

• ### Which of the following examples has stored potential energy within it?

a brick raised to a certain height

a coiled spring

a stretched rubber band

• ### As we carry a brick upwards the potential energy of that brick increases because _______.

the acceleration due to gravity increases

the mass of the brick increases

the distance from the ground increases

• ### A 5 kg rock and a 10 kg rock are dropped from a height of 10 m. Which of the following statements describes what happens to the potential energy of the rocks?

Neither rock loses potential energy because no work is done on the rocks.

The 10 kg rock loses more potential energy than the 5 kg rock because the 10 kg rock has more mass

Both rocks lose the same amount of potential energy because they fall from the same height.

The 10 kg rock loses more potential energy than the 5 kg rock because the 10 kg rock falls faster.

## ASK ANY QUESTION ON Energy

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##### Aayush

What is alternative energy?

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An electric bulb is marked with 60W.What does it mean?